The present invention relates to positive drives, and more particularly, to an improved clutch actuating mechanism for use in positive drives.
Although the improved clutch actuating mechanism of the present invention may be utilized advantageously in many different devices, it is especially advantageous when used in a traction modifying device, and more specifically, in a positive drive, and will be described in connection therewith.
Traction modifying devices have become popular for use in vehicles, in the drive train between the vehicle engine and the driven wheels. The primary function of a traction modifying device is to facilitate driving on slippery surfaces and in off-road conditions. These devices are especially useful where one of the driven wheels is momentarily subjected to worse traction conditions than the other driven wheel.
Traction modifying devices are generally categorized in three classes:
1. Limited slip differentials such as illustrated in U.S. Pat. Nos. 3,611,833 (nonbiased), and 3,624,717 (spring biased); PA0 2. Locking differentials such as illustrated in U.S. Pat. Nos. 1,111,728 (manually actuated), 2,978,929 (hydraulically operated), and 3,831,462 (speed responsive); and PA0 3. Positive drives such as illustrated in U.S. Pat. Nos. 1,477,311; 2,060,558; and 2,179,923.
During straight movement of a vehicle employing a positive drive, the engine power is transmitted approximately equally to the driven wheels, which rotate at the same speed. During a potential spin-out condition (e.g., one wheel on ice and the other on pavement), the engine power is transmitted to the driven wheels in proportion to their instantaneous traction limitations, whereby the wheels are still driven at the same speed.
When the vehicle turns a tight corner, power is transmitted only to the slower moving wheel, while the faster moving wheel is permitted to free wheel, relative to the input. When the vehicle is making a gradual turn, and the wheels are driven at slightly different speeds, the positive drive transmits slightly more torque to the slower turning wheel than to the faster turning wheel. Thus, a positive drive performs in a manner similar to an open differential during operating conditions which would make an open differential desirable, and performs in a manner similar to a rigid axle when operating conditions would make a rigid axle desirable.
The general construction and operation of positive drives has been known for over 40 years, as evidenced by the previously cited positive drive patents. A major disadvantage of prior art positive drives has been the use of large, complicated cam members, typically disposed between the opposed axles. As vehicles are continually downsized, it becomes more desirable to produce traction modifiers which are smaller and lighter. One way of accomplishing this objective is to eliminate the large central cam members and use a one piece housing and a single shaft member which comprises one of the cam members. Also, it is desirable to have inboard axle retention, such as "C" clips as used on most vehicle differentials, and this requires that the single shaft be removable. Such a design is illustrated and described briefly hereinafter, and is described in greater detail in copending application U.S. Ser. No. 355,971, filed Mar. 2, 1982, in the name of Edward J. Goscenski, Jr., and assiged to the assignee of the present invention.
One characteristic of a positive drive which distinguishes it from limited slip and locking differentials is that, in a positive drive, all of the axial force required to engage the clutch (locking the output to the input) is transmitted through the cam arrangement, whereas in limited slip and locking differentials, a substantial portion of the axial force is transmitted through the pinion gears and side gears. Thus, the engaging cam surfaces of a positive drive are subjected to greater loads and surface stresses, substantially increasing the potential for stress failure.
The problems associated with the greater axial forces in positive drives are magnified in the design of the type to which the present invention relates, in which the shaft is part of the cam arrangement. In such a design, it has seemed inevitable, prior to the present invention, that the engagement of the surfaces on the cam member and shaft would result in line contact, rather than surface contact during the active range of relative movement between the cam and shaft. If only line contact occurs, which under some conditions becomes only point contact, the large axial forces referred to previously will result in huge surface stresses over the very small amount of actual contact area and result in premature failure of the device.